System for pumping a fluid and method for its operation

1. A method of operating a system for pumping a fluid, the system including a pump comprising a suction side and a discharge side, a motor for driving the pump, the motor being drivingly connected to the pump via a shaft, a return line providing a feed-back conduit for the fluid from the discharge side to the suction side, and a control valve for controlling the flow of the fluid through the return line, the method comprising:

establishing a pump limit characteristics diagram by mapping a first system parameter (P 1 ) as a function of a second system parameter (P 2 ) to identify a permissible operating region of the pump, wherein the first system parameter (P 1 ) is a function of a differential pressure across the pump and the second system parameter (P 2 ) is a function of a pump torque;

for each first system parameter value, identifying a minimum allowable second system parameter value (P 2 0 );

monitoring the first system parameter (P 1 ) and identifying a minimum allowable second system parameter value (P 2 0 ) corresponding to the monitored first system parameter value (P 1 m );

monitoring the second system parameter (P 2 ) and comparing the monitored second system parameter value (P 2 m ) with the identified minimum allowable second system parameter value (P 2 0 ); and

regulating the control valve based on a relationship between the monitored second system parameter value (P 2 m ) and the minimum allowable second system parameter value (P 2 0 ) such that the monitored second system parameter value (P 2 m ) does not fall below the minimum allowable second system parameter value (P 2 0 );

wherein the system comprises a variable speed drive for operating the motor, and wherein the step of monitoring the second system parameter (P 2 ) comprises sampling the second system parameter (P 2 ) from the variable speed drive;

wherein the step of identifying a minimum allowable second system parameter value (P 2 0 ) comprises compensating the minimum allowable second system parameter value (P 2 0 ) for at least one of mechanical losses in at least one of the motor and the pump and electrical losses between the variable speed drive and the motor; and

wherein said mechanical losses are determined based on a rotational speed of the pump and said electrical losses are determined based on a power of the pump and the rotational speed of the pump.

2. The method according to claim 1 , wherein the first system parameter (P 1 ) is a differential pressure across the pump.

3. The method according to any one of claims 1 and 2 , wherein the second system parameter (P 2 ) is a torque (T) of the pump or a current (I) in windings of the motor.

4. The method according to claim 1 , wherein the step of regulating the control valve comprises opening the control valve when the monitored second system parameter value (P 2 m ) is within a predetermined range of the minimum allowable second system parameter value (P 2 0 ).

5. The method according to claim 1 , wherein said fluid is a hydrocarbon fluid.

6. The method according to claim 1 , wherein said step of establishing a pump limit characteristics diagram comprises establishing a pump limit characteristics curve for the pump, and wherein said step of identifying a minimum allowable second system parameter value (P 2 0 ) for each first system parameter value comprises establishing a pump operation curve in the permissible operating region, said pump operation curve being positioned at a predetermined distance from the pump limit characteristics curve.

7. A system for pumping a fluid, comprising:

a pump comprising a suction side and a discharge side;

a motor for driving the pump, the motor being drivingly connected to the pump via a shaft;

a return line providing a feed-back conduit for the fluid from the discharge side to the suction side;

a control valve for controlling the flow of the fluid through the return line;

a first sensor device for monitoring a first system parameter (P 1 ) which is a function of the differential pressure across the pump;

a second sensor device for monitoring a second system parameter (P 2 ) which is a function of a torque of the pump; and

a control unit which is arranged to:

receive monitored first system parameter values (P 1 m ) from the first sensor device and, for each monitored first system parameter value (P 1 m ), identify a minimum allowable second system parameter value (P 2 0 );

receive monitored second system parameter values (P 2 m ) from the second sensor device and, for each monitored second system parameter value (P 2 m ), compare the monitored second system parameter value (P 2 m ) with the identified minimum allowable second system parameter value (P 2 0 ); and

regulate the control valve based on a relationship between the monitored second system parameter value (P 2 m ) and the minimum allowable second system parameter value (P 2 0 ) such that the monitored second system parameter value (P 2 m ) does not fall below the minimum allowable second system parameter value (P 2 0 );

wherein the system comprises a variable speed drive for operating the motor, and wherein the control unit is arranged to monitor the second system parameter (P 2 ) by sampling the second system parameter (P 2 ) from the variable speed drive;

wherein the control unit is arranged to compensate the minimum allowable second system parameter value (P 2 0 ) for at least one of mechanical losses in at least one of the motor and the pump and electrical losses between the variable speed drive and the motor; and

wherein said mechanical losses are determined based on a rotational speed of the pump and said electrical losses are determined based on a power of the pump and the rotational speed of the pump.

8. The system according to claim 7 , wherein the minimum allowable second system parameter value (P 2 0 ) is identified using a pump limit characteristics diagram which maps the first system parameter (P 1 ) as a function of the second system parameter (P 2 ) to identify a permissible operating region of the pump.